| Au | |
|---|---|
| File extension: | .au |
| MIME type: | audio/basic |
| Magic: | .snd |
| Developed by: | Sun Microsystems |
| Type of format: | Audio container |
| Container for: | Audio, most often µ-law |
The Au file format is a simple audio file format introduced by Sun Microsystems. The format was common on NeXT systems and on early web pages. Originally it was headerless, being simply 8-bit µ-law-encoded data at an 8000 Hz sample rate. Hardware from other vendors often used sample rates as high as 8192 Hz, often integer factors of video clock signals. Newer files have a header that consists of six 32-bit words, an optional information chunk and then the data (in big endian format). Although the format now supports many audio encoding formats, it remains associated with the µ-law logarithmic encoding. This encoding was native to the SPARCstation 1 hardware, where SunOS exposed the encoding to apps through the /dev/audio interface. This encoding and interface became a de facto standard for Unix sound.
New format
| 32 bit word | field | Description/Content Hexadecimal numbers in C notation |
|---|---|---|
| 0 | magic number | the value 0x2e736e64 (four ASCII characters ".snd") |
| 1 | data offset | the offset to the data in bytes. The minimum valid number is 24 (decimal), since this is the header length (six 32-bit words) with no space reserved for extra information. |
| 2 | data size | data size in bytes. If unknown, the value 0xffffffff should be used. |
| 3 | encoding | Data encoding format:
|
| 4 | sample rate | the number of samples/second (e.g., 8000) |
| 5 | channels | the number of interleaved channels (e.g., 1 for mono, 2 for stereo, more channels possible but may not be supported by all readers) |
The type of encoding depends on the value of the 'encoding' field (word 3 of the header). Formats 2–7 are uncompressed PCM, therefore lossless. Formats 23–26 are ADPCM, which is a lossy, roughly 4:1 compression. Formats 1 and 27 are μ-law and A-law, respectively, both lossy. Several of the others are DSP commands or data, designed to be processed by the NeXT MusicKit software. Note: PCM data appears to be encoded as signed, rather than unsigned.
